#include <GL/glut.h>
#include <math.h>
#include <stdlib.h>
#include <stdio.h>

static GLint fogMode;

/*  Initialize depth buffer, fog, light source, 
 *  material property, and lighting model.
 */
static void init(void)
{
	GLfloat position[] = { 0.5, 0.5, 3.0, 0.0 };

	glEnable(GL_DEPTH_TEST);
	glLightfv(GL_LIGHT0, GL_POSITION, position);
	glEnable(GL_LIGHTING);
	glEnable(GL_LIGHT0);
	{
		GLfloat mat[3] = {0.1745, 0.01175, 0.01175};	
		glMaterialfv (GL_FRONT, GL_AMBIENT, mat);
		mat[0] = 0.61424; mat[1] = 0.04136; mat[2] = 0.04136;	
		glMaterialfv (GL_FRONT, GL_DIFFUSE, mat);
		mat[0] = 0.727811; mat[1] = 0.626959; mat[2] = 0.626959;
		glMaterialfv (GL_FRONT, GL_SPECULAR, mat);
		glMaterialf (GL_FRONT, GL_SHININESS, 0.6*128.0);
	}

   glEnable(GL_FOG);
   {
		GLfloat fogColor[4] = {0.5, 0.5, 0.5, 1.0};

		fogMode = GL_EXP;
		glFogi (GL_FOG_MODE, fogMode);
		glFogfv (GL_FOG_COLOR, fogColor);
		glFogf (GL_FOG_DENSITY, 0.35);
		glHint (GL_FOG_HINT, GL_DONT_CARE);
		glFogf (GL_FOG_START, 1.0);
		glFogf (GL_FOG_END, 5.0);
	}
	glClearColor(0.5, 0.5, 0.5, 1.0);  /* fog color */
}

static void renderSphere (GLfloat x, GLfloat y, GLfloat z)
{
	glPushMatrix();
	glTranslatef (x, y, z);
	glutSolidSphere(0.4, 326, 16);
	glPopMatrix();
}

/* display() draws 5 spheres at different z positions. */

void display(void)
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	renderSphere (-2., -0.5, -1.0);
	renderSphere (-1., -0.5, -2.0);
	renderSphere (0., -0.5, -3.0);
	renderSphere (1., -0.5, -4.0);
	renderSphere (2., -0.5, -5.0);
	glFlush();
}

void reshape(int w, int h)
{
	glViewport(0, 0, w, h);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	if (w <= h)
		glOrtho (-2.5, 2.5, -2.5*h/w, 2.5*h/w, -10.0, 10.0);
   else
		glOrtho (-2.5*w/h, 2.5*w/h, -2.5, 2.5, -10.0, 10.0);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity ();
}

void keyboard(unsigned char key, int x, int y)
{
	switch (key) 
	{
		case 'f':
		case 'F':
			if (fogMode == GL_EXP) 
			{
				fogMode = GL_EXP2;
				printf ("Fog mode is GL_EXP2\n");
			}
			else if (fogMode == GL_EXP2) 
			{
				fogMode = GL_LINEAR;
				printf ("Fog mode is GL_LINEAR\n");
			}
			else if (fogMode == GL_LINEAR) 
			{
				fogMode = GL_EXP;
				printf ("Fog mode is GL_EXP\n");
			}
			glFogi (GL_FOG_MODE, fogMode);
			glutPostRedisplay();
			break;
		case 27:
			exit(0);
			break;
		default:
			break;
   }
}

void main(int argc, char** argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(500, 500);
	glutInitWindowPosition (50, 50);
		glutCreateWindow("Fog : Press F or f for change Fog mode");
	init();
	glutReshapeFunc (reshape);
	glutKeyboardFunc (keyboard);
	glutDisplayFunc (display);
	glutMainLoop();
}